<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <select class="province">
        <option>江西省</option>
        <option>福建省</option>
        <option>湖南省</option>
    </select>
    <select class="city">

    </select>

    </div>
</body>
<script>
    let data = {
        江西省:["南昌市", "赣州市", "九江市"],
        福建省:["福州市", "厦门市", "泉州市"],
        湖南省:["长沙市", "湘潭市", "邵阳市"]
    };
    var provinceDom = document.querySelector(".province");
    var cityDom = document.querySelector(".city");
    provinceDom.onchange = function(){
        cityDom.innerHTML = "";
        for(var i = 0; i < data[provinceDom.value].length; i++){
            var createOption = document.createElement("option");
            createOption.text = data[provinceDom.value][i];
            cityDom.appendChild(createOption);
        }
    }
</script>
</html>